1. Field of the Invention
The present invention relates to a method for three dimensional measurement of objects being conveyed through a measurement frame for calculating the volume of the object by means of incremental measurements of height, width and length of the object. By volume is here considered both cubical volume and circumscribed volume.
2. The Prior Art
A system is known from U.S. Pat. No. 3,513,444 for determination of the volume of any object moving along a given path. The calculation of volume is effected by measuring height and width for each incremental distance which the object advances in the direction of conveyance. Upon each measurement there is thus calculated an incremental volume. A signal is provided to indicate that the object has moved the incremental distance in the direction of conveyance when a pulse generator, which may be connected to the drive system of the conveyer has provided a specific number of pulses being counted by a counter circuit. The total volume will thus be given the sum of the incremental volume measurements. This known system makes use of a number of parallel light rays in horizontal and vertical directions. Together the light rays form a right angled matrix. Opposite to the horizontal and vertical light sources, respectively, are located the same number of photocells. That number of photocells in the light matrix not receiving light, will thereby determine the height and width of the object at the actual location. The calculated volume is according to the patent, the real volume, whereas within the transport terminology one operates with cubical volume, defined by the product of the greatest height, length and width. The indicated solution does not provide storage of sufficient number of data to make it possible to calculate the cubical volume of the object in any one orientation thereof. The solution indicated by the U.S. Pat. has a further substantial limitation in that one dimension of that object which is to be measured always must be moved parallel to the direction of conveyance. This results in a substantial limitation with respect to utilization. U.S. Pat. No.3,588,480 and U.S. Pat. No. 3,436,968 disclose a system for retrieving data being representative of the volume of an object moving along a specified direction. For measurement of the dimensions of the object across the direction of movement, there is used a larger number of light emitters being mounted to form a matrix of parallel light rays. The dimension of the object in the direction of movement is determined by measuring its velocity and the time it is present within the light matrix. Constant light is emitted the whole time from the light matrix. According to the patents the systems thus described are able to measure both the real and cubical volume of the object. The cubical volume is derived by multiplication of the maximum linear dimensions in each direction of the object. In order to enable this in a simple manner with the described electronics, it is required that the object is orientated with its largest length/width parallel to/across the direction of movement. It is indicated that the orientation of the object on the conveyor is immaterial per se, however, without indicating how the volumes in such case are calculated.
Swedish Pat. No. 425,126 discloses a method for the detection of the presence of an object within a measurement area, and measurement of its cross section in the measurement plane. For accurate measurement of dimensions it is assumed that the crossection of the object is approximately circular, e.g. a log. The measurement apparatus includes a device consisting of two arrays each having a number of transmitters and receivers. The measurement area is defined as the plane between the two arrays. Each array consists of a number of transmitters and a smaller number of receivers. If the distance between the receivers is D and there are the number s of transmitters between each receiver, it is possible to obtain a measurement accuracy of .+-.D/2s. By means of a described dual receiver, the accuracy is claimed to increase to .+-.D/4s. The disclosed geometry with larger distances between the receivers than between the transmitters renders the measurement system completely unsuitable for objects having a crossection departing substantially from that of a circle.
The present invention aims at providing an improved method for three dimensional measurement, rendering it possible to calculate therefrom both cubical volume and circumscribed volume, irrespective of the orientation of the object on the conveyor belt. The present invention aims in particular to provide a solution whereby an automatic adaptive calibration of the measurement system, automatic error detection and signal processing with improved measurement accuracy are enabled. In addition, the invention aims at the use of system components having relative modest requirements to tolerance values.
As a further object of the invention there is intended to provide modular construction of the measurement frame, in order that the present invention enables simple "tailor-made" solutions for dimension measurement of objects within the different volume ranges. Such modular construction will also facilitate trouble-shooting and repairs.